Electrophotographic color reproductions are conventionally made by forming monocolor toner images of different color on an image member and transferring those images in registration to a single receiving sheet. The receiving sheet is held by a transfer drum, usually with gripping fingers, which is rotated to bring the receiving sheet repetitively into transfer relation with the image member in a nip to overlay the toner images. Transfer is accomplished by an electric field in the nip having a direction urging the toner to move to the surface of the receiving sheet.
The field in the nip attracts the toner to the paper. At the same time, the field causes the paper to be attracted to the image member, which contributes to forces tending to cause the paper to follow the imaging member rather than the transfer drum.
Once the paper has been intimately held by the transfer drum, the paper can become electrostatically attracted to the drum and be difficult to remove. These competing forces vary with temperature and humidity. Thus, the industry has found great difficulty in controlling the paper in color transfer apparatus of this type, especially apparatus designed to operate in varying conditions over long runs with no paper jams. The industry approaches this difficulty by feeding the paper into contact with the drum well prior to the nip and gripping the paper with small fingers forming part of the drum to hold the paper securely. The fingers hold the paper until all transfers have been made and the paper has left the nip for the last time. At that point the fingers release the paper and paper separating skives separate the paper from the transfer drum. Although this approach has the advantages of reasonable certainty in holding the paper and releasing the paper, the gripping fingers on the transfer drum add complexity and the skives have a tendency to wear the drum.
Some color systems do not lend themselves to the use of gripping fingers at all. For example, U.S. Pat. No. 4,712,906, Bothner et al, issued Dec. 15, 1987, shows an electrophotographic color printer which forms consecutive images in different colors that are transferred in registry to a receiving sheet. The receiving sheet is wrapped around a transfer drum and recirculated on the surface of the drum into transfer relation with the consecutive images to create a multicolor image on the sheet. To improve efficiency, large sheets, for example "ledger" size sheets are placed on the drum with the small dimension parallel to the axis of the drum and wrapped substantially around the transfer drum. Small sheets, for example, "letter" size sheets, are placed with their long dimension parallel to the axis of the drum. Since the short dimension of letter size sheets is half the long dimension of ledger size sheets, two letter size sheets are placed on the drum in approximately the same space as the single ledger size sheet.
The Bothner invention is difficult to utilize with gripping fingers because the leading edge of the second letter size sheet is positioned at approximately the middle of a ledger size sheet. For some applications, retractable fingers may be made to work, but for many applications they would leave substantial image artifacts in a ledger size sheet. Bothner therefore suggests the use of vacuum holes which are positioned at the leading edge of each of the smaller sheets and may or may not both be activated for the ledger size sheet.
The Bothner structure, as described, works well for most environments. However, in some temperature and humidity conditions found in some locations difficulty is encountered both with initial pickup by the transfer drum of the transfer sheet and release of the transfer sheet from the transfer drum as the last image is being transferred.
U.S. Pat. No. 4,674,860 to Tokunaga et al issued June 23, 1987 shows a transfer drum to which a receiving sheet is tacked electrostatically by spraying electrostatic charge on either the sheet or the drum or both. The bias on the transfer drum is switched between positive and negative to initially attract the sheet which has been charged and later to attract the toner to the sheet.
U.S. Pat. No. 4,740,813 to Roy issued Apr. 26, 1988 shows a transfer drum using vacuum holes in which the vacuum portion of the drum is not biased when in the nip to aid in the location of the leading edge and trailing edge of the receiving sheet.
Prior filed U.S. patent application Ser. No. 430,037 now U.S. Pat. No. 5,040,029, to Rodenberg et al, filed Nov. 1, 1989, describes a method of forming multicolor toner images in which the leading edge of the receiving sheet is attracted by a vacuum to the transfer drum. Immediately upon attraction of the leading edge to the drum, the transfer field is applied. In a preferred embodiment the transfer field is removed as the leading edge passes through the nip for subsequent transfers to prevent it reattaching to the photoconductor. As the last image approaches the transfer nip, the field is maintained as the vacuum is released and the transfer sheet follows the image member rather than the transfer drum from which it is later separated.